The present invention relates to improvements in fluid power load-clamping systems for variably regulating maximum load gripping forces in a manner automatically adaptive to at least one characteristic of the load.
Various types of such adaptive load-clamping systems have been proposed in the past. Such previous systems can be categorized as follows:
(1) Systems which sense the existence of load slippage and respond automatically by gradually increasing the gripping force on the load by fixed force increments until the sensed slippage stops;
(2) Systems which automatically vary the gripping force in proportion either to the sensed weight or to the resistance to gripping of the load, without regard to whether or not slippage is actually occurring; and
(3) Systems which perform a combination of (1) and (2).
Fluid power clamping systems of any of the above types regulate gripping force by gradually increasing gripping fluid pressure automatically from a relatively low threshold pressure. However such low threshold pressure limits the speed with which the load-engaging surfaces can be closed into initial contact with the load, thereby limiting the productivity of the load-clamping system. This problem occurs because highspeed closure requires higher closing pressures than the desired low threshold pressure, such higher pressures becoming trapped in the system by fluid input check valves during initial closure so that the desired lower threshold pressure is exceeded before automatic regulation of gripping pressure can begin. Although gripping pressure relief valve systems have in the past provided high and low relief settings selectable either manually, or automatically in response to clamp closure speed, to enable high-speed closure followed by low maximum gripping pressure, no such systems capable of automatically changing such settings in a manner compatible with automatic variable gripping pressure regulation have been known.
Prior fluid power systems such as those disclosed in British Patent Publication No. 2312417 and German Patent Publication No. 3245715, which vary the gripping fluid pressure in proportion to the sensed weight of the load, obtain weight measurements by lifting the load. However such weight-sensing systems are susceptible to inaccurate weight measurements due to variations in the speed with which the load is raised during weighing. The system shown in the British publication is also susceptible to inaccurate weight measurements due to variations in lifting pressure which are inherent within the extensible lifting mechanism depending upon its degree of extension. Furthermore, such prior systems do not weigh the load in response to tilting of the load which, in the operation of industrial lift trucks and especially in paper roll handling operations, is a commonly-used alternative way to lift the load.
Prior fluid power weight-responsive gripping pressure regulation systems normally make such pressure regulation responsive to clamp closure actuation. However, particularly for systems on industrial lift trucks, lifting and weighing the load in response to clamp closure actuation is counterintuitive and confusing for the operator, who is accustomed to lifting the load in response to a lifting actuator.
Such prior weight-responsive systems also do not provide for different selectable predetermined relationships between the weight of the load and the gripping pressure, which are needed to account for variations in load fragility and stability.
Although automatic load tilt adjustment systems have been provided in the past for leveling fragile loads to prevent edge damage when the load is being set down, such automatic adjustment systems have not been capable of sensing the tilt of the load with respect to gravity, leading to inaccurate automatic tilt adjustment when an industrial lift truck is not level with respect to its supporting surface, or such surface is not level.
Valves for automatically preventing excessive lowering of the lifting mechanism when a clamped load is set down, to prevent subsequent damage to fragile load surfaces by downward slippage of the clamp when it is opened to disengage the load, have been provided in the past as shown, for example, in U.S. Pat. No. 3,438,308. However, such previous systems lack the versatility needed for reliable protection of the load under variable circumstances, such as variations in the degree of extension of the lifting mechanism when the load is set down.
In one preferred aspect of the invention, a controller automatically enables high initial clamp closure speed prior to automatic gripping pressure regulation by initially permitting relatively high fluid pressure to close the clamp, followed by an automatic reduction in the maximum fluid pressure as the clamping surfaces close into a predetermined relationship with the load, followed by an increase in the maximum fluid pressure pursuant to automatic maximum gripping pressure regulation.
In another separate preferred aspect of the invention, a limit on the rate of lift of the load is automatically imposed during weighing, to maximize the accuracy of the load-weight measurement.
In another separate preferred aspect of the invention, the load-weight measurement is compensated to account for variations in extension of the lifting mechanism, also to maximize the accuracy of the load-weight measurement.
In another separate preferred aspect of the invention, automatic gripping pressure regulation is selectively operable in response to actuation of a lift selector.
In another separate preferred aspect of the invention, automatic gripping pressure regulation is selectively operable in response to actuation of a tilt selector.
In another separate preferred aspect of the invention, different predetermined relationships between the weight of the load and the maximum gripping pressure are selectable alternatively.
In another separate preferred aspect of the invention, a gravity-referenced tilt controller automatically adjusts the load to an attitude which is untilted with respect to gravity.
In another separate preferred aspect of the invention, an improved system is provided for automatically preventing further lowering of the lifting mechanism when the load is set down, until the clamp has been opened to disengage the load.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description, taken in conjunction with the accompanying drawings.